1. Field of the Invention
The present invention relates to a functionally gradient member constructed in an integrated manner by a ceramic part composition layer and a metal part composition layer. The present invention also relates to a method for producing the functionally gradient member, and a semiconductor circuit substrate based on the use of the functionally gradient member.
2. Description of the Related Art
In general, the semiconductor circuit is devised such that the semiconductor circuit is carried on a ceramic insulating substrate to efficiently discharge the heat generated in the semiconductor circuit to the outside, in order to stabilize semiconductor characteristics.
Especially, in recent years, it is intended to make large electric power control, high speed switching, and high degree of integration, in which there is a notable tendency to achieve high degree of integration, large capacity, and high frequency of the semiconductor circuit. Therefore, it is demanded to reliably perform heat radiation in order to avoid occurrence of, for example, decrease in circuit performance, runaway control, malfunction, and circuit breakdown which would be caused by the heat generated in the semiconductor circuit. Accordingly, an artifice is devised to braze or solder a heat sink or a heat sink base made of copper or aluminum onto the ceramic insulating substrate.
Specifically, copper is metallized on one surface of the ceramic insulating substrate, and then nickel plating is formed thereon, in order to improve the wettability of solder or the like upon mounting of the semiconductor chip. Copper is metallized in the same manner as described above on the other surface of the ceramic insulating substrate, onto which a heat sink or a heat sink base is joined through nickel plating and solder. Thus, a multiple layered structure is formed as a whole.
The ceramic insulating substrate is required to have high thermal conductivity in order to maintain characteristics of the semiconductor circuit at high levels. Further, the ceramic insulating substrate is required to have insulation property, shielding property, and low dielectric property. On the other hand, the heat sink is also required to have high thermal conductivity. Usually, both of the ceramic insulating substrate and the heat sink are designed to have a thermal conductivity of not less than 150 W/mK and have a coefficient of thermal expansion which is approximate to the coefficient of thermal expansion of the semiconductor chip.
However, the brazing material or the soldering material, which is used to join the ceramic insulating substrate and the heat sink, has a coefficient of thermal expansion which is two-fold or more as compared with those of the ceramic insulating substrate and the heat sink, and a coefficient of thermal conductivity which is not more than 20 W/mK to 70 W/mK. That is, the brazing material or the soldering material has a low value of the coefficient of thermal conductivity which is 1/2 to 1/7 as compared with those of the ceramic insulating substrate and the heat sink. For this reason, the joining section, at which the brazing material or the soldering material is applied, has a low coefficient of thermal expansion. Further, the joining section undergoes large thermal expansion as compared with other sections. A problem is pointed out that a considerably large stress is generated at the joining section, and the reliability of joining is lowered.
Moreover, it is also feared that the heat tends to be accumulated in the joining section, and it is impossible to effectively exhibit the function of the heat sink. Therefore, it is necessary to provide a considerably large heat sink and a considerably large radiating fin so that a large thermal gradient is always maintained. As a result, a problem arises in that such an arrangement cannot respond to the requirement of miniaturization.
Additionally, the following problems arise. That is, the operation performance of the circuit is deteriorated as the temperature is raised. For example, there are caused deterioration of the transistor function such as the delay of switching speed and the decrease in current and voltage magnification. Further, circuit breakdown occurs due to thermal runaway, and there are caused, for example, peeling at the joining section and peeling of the mounted chip due to heat.